In prior art devices, proportional flow or pressure control has been performed by partially closing or opening a valve to control the restriction of the valve, and thus the flow through the valve and the pressure drop across the valve. In the case of a microvalve, which is electrostatically actuated, it is not possible to perform proportional control in this manner due to the electrostatic pull-in effect when the critical field is reached in the valve structure.
In certain designs for microvalves and arrays, proportional control of flow and pressure has been attempted using a pulse-width modulation technique at a frequency ranging from approximately 20Hz to several hundred Hz, to control the amount of time the valve is open and closed. This pulse-width modulation technique requires constant operation of the valve arrays, resulting in a lifetime operating requirement for the number of actuations for the valves to be in the two to twenty billion range. Such a requirement for a proportional control valve is extremely difficult to achieve, if it is indeed possible with today's technology.
One example of a miniature gas valve is shown in commonly owned U.S. Pat. No. 5,082,242, divisional U.S. Pat. No. 5,180,623, and further divisional U.S. Pat. No. 5,244,537. These patents, incorporated by reference herein, describe an electronic microvalve in which the operative voltage is minimized. The microvalve is an integral structure made on a single piece of silicon and is a flow through valve with inlet and outlet on opposite sides of the silicon wafer.
Examples of microstructure gas valve control are shown in U.S. Pat. No. 5,176,358, divisional U.S. Pat. No. 5,323,999 and further divisional U.S. Pat. No. 5,441,597. These patents, also incorporated by reference herein, describe a flow control device in which there are separate main flow and flow control (servo) passages between an inlet port and an exit port. The valves are formed on a monolithic semiconductor and are electrostatically controlled. U.S. Pat. No. 5,822,170, also incorporated by reference herein, describes in greater detail a touch mode electrostatic actuator which is useful in the above described valves, and which would be subject to a lifetime operating requirement for the number of actuations for the valves to be in the two to twenty billion range.
Accordingly, it would be of great advantage in the art if the usable lifetime of a valve array for a proportional flow or pressure control application would be greatly increased.
It would be another great advance in the art if more precise control of electrostatic arrays could be achieved, particularly in the manner in which separate valves in an array are actuated.
Other advantages will appear hereinafter.